1. Field of Invention
This invention relates to a method and apparatus for determining, by ion chromatography, the content of a microconstituent in a major constituent, such as impurities in a main component.
2. Description of the Prior Art
The term "ion chromatography" is used to describe high speed liquid chromatography, as suggested in a publication by H. Small et al in 1975, and is used mainly for the analysis of inorganic ions. This method has been widely accepted and is now utilized in various forms of trace analysis including the analysis of environmental samples.
FIG. 1 illustrates a conventional ion chromatograph which embodies the ion chromatography process. In the diagram, an eluate tank 1a is used for storing an eluate, which is an aqueous solution containing Na.sub.2 CO.sub.3 /NaHCO.sub.3 in a concentration on the order of several mM/liter. A pump 2 forwards the eluate under pressure to a sample inlet device 3 to be described more specifically hereinafter. Sample inlet device 3 is used for admitting (or automatically collecting) a prescribed amount of sample solution, introduced, for example, by a microsyringe, and forwarding the sample solution with the eluate from pump 2, to a separation column 4 packed, for example, with an anion-exchange resin. Column 4 is connected to a cation removing device 5 comprising two adjoining chamber, i.e. a first chamber for passing the effluent originating in separation column 4 and flowing through a wall of a perfluorocarbon sulfonate type cation exchange composition, such as for example, NAFION (proprietary designation of DuPont Company), and a second chamber for passing a scavenger liquid to be described in more detail hereinbelow. A detector 6 is used for passing the effluent from the first chamber of cation removing device 5 and, at the same time, for measuring the electric conductivity of the effluent. A recording meter 7 receives the output signal of detector 6 and describes a chromatogram in response to the output signal. A scavenger liquid tank 1b stores a scavenger liquid formed of a prescribed solvent such as, for example, dodecylbenzene sulfonic acid, which is pumped by pump 9 to forward the scavenger liquid from scavenger liquid tank 1b, under pressure, to the second chamber of the cation removing device 5, and then to a tank 8b for storing the scavenger liquid flowing out the second chamber of device 5. A tank 8a stores the liquid which has undergone measurement in detector 6 after flowing out of detector 6. Usually separation column 4, cation removing device 5, and detector 6 are disposed in a constant temperature bath 10 which is maintained at a prescribed temperature.
In the ion chromatograph above described, when the eluate, Na.sub.2 CO.sub.3 /NaHCO.sub.3, held inside eluate tank 1a is introduced via pump 2, sample inlet device 3, and separation column 4 into cation removing device 5, it is converted therein into H.sub.2 CO.sub.3 through cation exchange of Na.sup.+ to H.sup.+. As a result, the electric conductivity of the eluate is lowered and the output signal background of detector 6 is eventually lowered. When the solution to be measured is taken in the prescribed amount and is introduced through sample inlet device 3, it is conveyed by the eluate, taken from reservoir 1a, to separation column 4. Here, the ion species, such as for example, an anion, in the solution to be determined, is chromatographically separated. The effluent from column 4 is again conveyed by the eluate via cation removing device 5 into detector 6. Detector 6 consequently feeds out a signal corresponding to the ion species present in the solution being determined.
When the ion chromatograph of the above description is utilized for the determination of a microconstituent (such as, for example, an anion) present in the solution being tested and when the solution contains a major constituent in a very large concentration besides the microconstituent, the efficiency with which the two constituents are separated from each other is degraded even to the extent of rendering the determination of the microconstituent extremely difficult. To overcome the difficulty, there has been made an attempt to effect the determination of the microconstituent by the use of a so-called selective detector which responds only to the microconstituent and not to the major constituent (hereinafter called "first conventional method").
Also, another suggested solution was to attain the determination of the microconstituent by subjecting the solution being tested to a pretreatment for removing the major constituent from the solution and subjecting the remaining solution to analysis (hereinafter called "second conventional method").
Separately, a so-called fractionation and reintroduction method has been attempted to resolve the problem above mentioned. The method comprises analyzing the solution being tested by an ion chromatograph, fractionating the portion of the effluent from a detector corresponding to the neighborhood of the microconstituent while consulting the signal from the detector, subsequently injecting the fraction with the aid of a syringe into a concentration column, and determining the content of the micrconstituent again by the use of the ion chromatograph (hereinafter called "third conventional method").
Unfortunately, the first and second conventional methods are disavantageous in that they lack general adaptability and have been proven to be practicable only when microconstituents to be determined are specific substances. In the case of the third conventional method, since the amount of the fractionated solution is generally large, the method necessitates use of a concentration column and the resistance offered to the eluate flowing through this concentration column is generally large. In this connection, a suppressor using an ion exchange membrane is deficient in resistance to pressure and, therefore, is easily breakable. Thus, the concentration column which experiences such heavy resistance to the solution cannot be connected to the downstream side of the suppressor. Thus, the third conventional method has suffered from a further disadvantage in that desired automation of the determination by interconnecting a solution collecting valve possessed of the concentration column and a flow path switch valve disposed on the downstream side of the detector through the medium of a connecting pipe is extremely difficult to accomplish.